The present invention relates to a silicon nitride sintered material having a high-temperature strength, excellent thermal shock resistance and a low Young's modulus, as well as to a process for producing the sintered material.
With respect to silicon nitride sintered materials containing oxides of IIIa group elements including rare earth elements, for example, Japanese Patent Publication No. 7486/1973 discloses a process for producing a sintered material, which comprises mixing and shaping 85 mole % or more of silicon nitride (Si.sub.3 N.sub.4) and 15 mole % or less of at least one oxide selected from oxides of IIIa group elements and then sintering the shaped material in a non-oxidizing atmosphere. Japanese Patent Publication No. 21091/1974 discloses a silicon nitride sintered material consisting of 50% by weight of Si.sub.3 N.sub.4, 50% by weight or less of at least one oxide selected from Y.sub.2 O.sub.3 and oxides of La type elements and 0.01-20% by weight of Al.sub.2 O.sub.3.
However, there have been problems that addition of only rare earth elements to silicon nitride fails to provide a sintered material having a high-temperature strength and addition of Al.sub.2 O.sub.3 provides a sintered material which has a higher density but whose grain boundary phase has a lower melting point and gives a very low high-temperature strength.
In order to solve the problem of inadequate high-temperature strength, Japanese Patent Application Kokai (Laid-Open) No. 100067/1988 discloses a technique for achieving a high-temperature strength by adding rare earth elements of given composition and given proportion to a Si.sub.3 N.sub.4 powder and sintering the mixture to allow the resulting sintered material to have a specific crystalline phase.
The silicon nitride sintered material disclosed in Japanese Patent Application Kokai (Laid-Open) No. 100067/1988 can achieve a high-temperature strength to some extent but has problems that the Young's modulus is as large as 300 GPa and the thermal shock resistance .DELTA.Tc (.degree.C.) is as small as 1,000.degree. C. This is because the silicon nitride sintered material is homogeneous microscopically, and has a Young's modulus and thermal expansion coefficient characteristic of silicon nitride and, as a result, the thermal shock resistance .DELTA.Tc (.degree.C.) of the silicon nitride sintered material is substantially determined depending upon its strength. The thermal shock resistance .DELTA.Tc (.degree.C.) of a ceramic can be evaluated by the following formula. EQU .DELTA.Tc.varies..sigma./.alpha.E
[.sigma. is a flexural strength (Pa), .alpha. is a thermal expansion coefficient (1/.degree.C.), and E is a Young's modulus (Pa).]
The object of the present invention is to solve the above-mentioned problems and provide a silicon nitride sintered material having a high-temperature strength about equal to the room temperature strength and excellent thermal shock resistance, as well as a process for producing the sintered material.